Me and my father just tinkered around with my Sony Walkman NWZ-S544 because it was stuck permanently on hold rendering it basically useless...
We were able to fix this problem and I wanted to pass it on to other people!
Now this is only a good solution if you're past your warranty, because tampering with it will void it. Also if you're not very tech-savy it's prolly a good idea to get someone to help you. *This solution will also make it impossible to put your mp3 player back on hold as it removes the switch*

After opening the walkman enough to access the hold switch (a little silver box with a black tab sticking out of it) You can pry off the thin metal cover with a small flat screwdriver. Gently pry it back and forth till it comes off, you will not need this piece anymore. Remove the small black tab(it also has a small strip of metal on the bottom of it) and throw this away if you also wish. Put everything back together and it should work!!